Curable, or thermosettable, coating compositions are widely used in the coatings art, particularly for topcoats in the automotive and industrial coatings industry. Color-plus-clear composite coatings provide topcoats with exceptional gloss, depth of color, distinctness of image, and special metallic effects. The automotive industry has made extensive use of these coatings for automotive body panels. A topcoat coating should be durable to maintain its appearance and provide protection under service conditions during the lifetime of the coated article. Topcoat coatings for automotive vehicles, for example, are typically exposed to all kinds of weather, ultraviolet rays from the sun, abrasions from gravel thrown up during driving or from items set on the car when parked, and other conditions that can degrade the coating. For some time, researchers have directed their efforts to providing coatings with greater resistance to environmental etch. “Environmental etch” is a term applied to a kind of exposure degradation that is characterized by spots or marks on or in the finish of the coating that often cannot be rubbed out.
Curable coating compositions utilizing carbamate-functional resins are described, for example, in U.S. Pat. Nos. 5,693,724, 5,693,723, 5,639,828, 5,512,639, 5,508,379, 5,451,656, 5,356,669, 5,336,566, and 5,532,061, each of which is incorporated herein by reference. These coating compositions can provide significant improvements in resistance to environmental etch over other coating compositions, such as hydroxy-functional acrylic/melamine coating compositions. On the other hand, carbamate-functional resins tend to require more organic solvent to achieve acceptable viscosity for application and leveling of the applied film to obtain desired smoothness. Coatings with higher amounts of organic solvent produce more regulated emissions during application. Coatings with hydroxyl-functional acrylic polymers cured using blocked polyisocyanate can also provide excellent resistance to environmental etch in cured coatings, but these coatings do not have the desired scratch and mar resistance. Coatings with hydroxyl-functional acrylic polymers cured using aminoplasts can be formulated at higher solids and cured at lower temperatures relative to the other compositions mentioned, but do not provide the environmental etch resistance or scratch and mar resistance of the other coatings. Other coating chemistries have been used, but these also have shortcomings, such as poor weathering properties or high volatile organic content [VOC].
It would be advantageous to have a coating composition that could provide desired environmental etch resistance and improved scratch and mar resistance without dramatically increasing the viscosity of the coating composition.